This invention relates to a signal switching output device for television broadcasting or in a system device such as a VTR, a video disk player, or a compact disk player, for instance, to overlay (superimpose) characters based on an RGB signal such as an RGB image signal making use of the subcode of a compact disk over an image reproduced by a video disk player, for instance, on a display unit.
Sometimes characters such as time or telop are inserted, or characters are overlaid from a personal computer or according to an RGB signal stored in the subcode of a compact disk onto a TV screen or on a portion of an image based on image signals generated by a video disk player.
In overlaying a character over an image which is a background, if the character is white and the background image is bright and whitey, it is very hard to recognize the character.
As a method for solving a problem like this, a brightness level of image signals for the background other than the character is attenuated, and then character signals consisting of RGB signals are hybridized for overlaying the character over the background image.
Also, as another method for making an overlaid character easier to be recognized, a rectangular frame is arranged at a portion of a display screen where characters are displayed, and a color of an area inside the frame is kept black or grey, or a brightness level of a background image inside the rectangular frame is made lower than that of a surrounding area (Japanese Patent Publication No. 2-35510).
In the method for making the screen to be dark by lowering a brightness level of a background image described above, a displayed background image is darker than the original image, so that a viewer may recognize it as a different image. By lowering the signal level, also the level of synchronizing signals composing said image signal and the level of the color burst signal are attenuated. Then, since an hybridized image signal for the background image is used, as an output to a display unit, together with a synchronizing and a color burst signal, if the attenuation factor is too large, the possibility of causing a problem related to the operations of the display unit is high. Moreover, as the level of the image signal is attenuated, drop of S/N is inevitable.
Also in the latter method, wherein a black or grey rectangle is arranged at a portion of a screen, an image at the portion is hidden by the rectangle, which gives discomfort to users. Also in the case, wherein a brightness level of an image in the rectangle is made lower than that of an image surrounding the rectangle, a similar problem occurs.